The present invention relates to semiconductor devices with buried contacts and, more particularly, to textured polycrystalline buried contact solar cells.
Polycrystalline and monocrystalline (single crystalline) wafers are useful substrates to fabricate solar cells. Altering the surface topography of solar cells, such as texturing the front surface of solar cells, is useful to trap light, reduce light reflection, and increase efficiency. A large amount of light is coupled into the silicon at very oblique angles and can provide an effective increase in the diffusion length of the substrate with many more carriers being generated within close proximity of the junction. Various techniques have been suggested to provide textured surfaces with pyramids or grooves for wafers and cells. Surface texturing helps redirect reflected light into the cell, reduces grid shadowing, and decreases the reflectance of the active area. Micro texturing can be accomplished, such as by chemical etchants or photolithography, to form small pyramids, inverse pyramids or micro grooves. The results of chemical etchants depend on crystal orientation. Chemical etchants can be useful on monocrystalline (single crystalline) wafers but are not generally useful with polycrystalline silicon with its random orientation of crystal grains. Other techniques can be used for texturing such as laser cutting.
Mechanical texturing increases the current generating capability of buried contact solar cells. Mechanical texturing or macro texturing can be accomplished, such as with a dicing saw, to form larger pyramids or macro grooves. Dicing saws and beveled blades are useful for V-groove formation. Macro texturing provides the best results for polycrystalline wafers which generally do not benefit from conventional chemical texturing. Macro texturing has also yielded cells with higher short circuit current density than that of chemically textured buried contact cells.
Forming macro grooves and metallization grooves by sawing rather than by use of a laser is desirable to form improved semiconductor devices. Where a laser is used, although the grooves need not extend to the edge of the device, the laser must melt the silicon or other material of the wafer causing the groove to, be wider than desired. Wide grooves undesirably result in a less active area for the solar cells. It is desirable to cut grooves as narrow as possible to minimize shadowing. A diamond based dicing saw creates deeper, narrower grooves, which are preferred in the production of semiconductor devices, particularly solar cells. Furthermore, forming grooves by using a laser is relatively slow. Cutting with a saw, particularly a dicing saw, cuts the semiconductor material of the wafer faster than with a laser. During the use of a laser the wafer tends to conduct heat away from the area being melted and silicon is a good heat conductor. Moreover, it is convenient and less expensive to add multiple saw blades to a cutting or milling machine to form parallel grooves, than to attempt to employ multiple laser beam devices.
While macro texturing improves the short circuit current density of the cells and increases the effective surface area of the emitter, it also creates high areas of stress and decreases the open circuit voltage V.sub.oc over non-textured cells. Furthermore, macro texturing does not provide a smooth channel of uniform depth for top contact metallization which is necessary to provide electrically conductive contacts but instead causes wavy, zig-zag, or sinusoidal, metal contacts of varying depth. This results in significant higher metallization (plating) costs, greater production time to accommodate more metallization, and increases the volume of metallization and the area of the cell to be in contact with the metal compared with non-textured planar cells.
Macro texturing also increases the metal-silicon contact area which reduces the open circuit voltage of the solar cell. For a solar cell of 11.4 cm .times.11.4 cm, there are about 2000 macro grooves and 90 metallization fingers (contacts). The surface area of the contact between the heavily diffused regions and the metallization increases by about 18% due to the serpentine pattern.
Buried contacts can be used in solar cells and other semi-conductor devices to enhance efficiency. The metallization grooves cut for buried contacts are preferably cut by sawing rather than with a laser for the reasons described above. When conventional buried contact wafers made by sawing are cut or otherwise separated into solar cells, however, the buried contacts are often cut, which short or shunt the solar cells rendering the solar cells useless.
It is, therefore, desirable to provide improved macro textured buried contact solar cells and other semi-conductor devices which overcome most, if not all, of the preceding problems.